There are prior arts, for example, those disclosed in Taiwanese Utility Model Patent Nos. M534055 and M541362, which provide a machine spindle having a collet to clamp a pull stud of a tool holder and to thereby position a tapered end of the tool holder assembly to the machine spindle.
Referring to FIGS. 1 and 2, when a collet 101 of a machine spindle 1 is loosened and placed in an open state, and a tool holder assembly 2 extends into the machine spindle 1, a pull stud 201 of the tool holder assembly 2 abuts a drawbar 102 of the machine spindle 1 but is not clamped by the collet 101. The pull stud 201 has an enlarged connection head 202.
As shown in FIG. 3, when the collet 101 is moved upward and tightened, the enlarged connection head 202 is clamped by the collet 101 and a spacing (S) is formed between the enlarged connection head 202 and the drawbar 102.
In general, the tool holder assembly 2 is detachably connected to the machine spindle 1. The machine spindle 1 and the tool holder assembly 2 assembled together are commonly applied in industries, such as aerospace industries, to process thin workpieces at high speed. The processing operation of the tool holder assembly 2 requires a cryogenic fluid to cool down the workpieces for preventing the workpieces from being melted and becoming sticky. For example, in a low-temperature cutting process, processing temperatures are divided into three zones, a first zone (2-6° C.), a second zone (0-30° C.) and a third zone (<−50° C.). The cryogenic fluid is generally liquid nitrogen. The cryogenic fluid is delivered from the machine spindle 1 to the tool holder assembly 2 passing through the drawbar 102. Because the enlarged connection head 202 and the drawbar 102 has the spacing (S) therebetween, which has a cross section larger than flow passages (P1, P2) of the connection head 202 and the drawbar 102, the cryogenic fluid flowing through the spacing (S) can encounter a pressure drop in the spacing (S) (FIG. 3). Due to the pressure drop, the cryogenic fluid is gasified as shown by arrows in FIG. 3, thereby causing a change or an increase in temperature of the cryogenic fluid reaching the tip of a cutter (not shown) which adversely affects the efficiency of low-temperature cutting.
If the existing tool holder assembly of the type as described above is applied in aerospace industries, it may cause workpieces to melt during its processing operation.